1. Field of the Invention
The present invention relates to a power supply circuit used in a liquid crystal display device or the like, more specifically to a power supply circuit with a high drive performance having mixed loading of a liquid crystal display drive, a control circuit, RAM and the like.
2. Description of the Related Art
In recent models of a mobile device typified by a mobile telephone and the like, types of the device, to which a power supply circuit for supplying power required for a plurality of functions is mounted, is increased. Such a model is advantageous in the point that power consumption and number of power-supply lines can be reduced by controlling on and off of the power supply in concert with on and off of the function.
As this type of the power supply circuit, the power supply circuit has been conventionally known which could realize a phase stability of an output from an operational amplifier and response to a momentary change of the output by connecting a smooth capacitor to an output terminal of the operational amplifier via a resistance (for example, see No. 2002-14733 of the Japanese Patent Applications Laid-Open).
As another example is known a power supply circuit which could realize a phase stability and high-speed operation by automatically adjusting a phase compensation resistance and a capacitance in the operational amplifier without using the smooth capacitor (for example, see No. H11-340753 of the Japanese Patent Applications Laid-Open).
FIG. 48 shows a conventional power supply circuit AO1 for assuring area reduction, less power consumption and phase stability. In FIG. 48, 1 denotes an input terminal of a reference potential Vin, 2 denotes a power supply circuit output terminal, B1 denotes an operational amplifier for generating a power supply potential V1 by buffering the reference voltage Vin, R01 denotes a phase stability resistance, and E1 denotes a smooth capacitor. The resistance R01 generally has an impedance in the range of a few Ω-a few-hundred Ω order in order to stabilize a phase of the output voltage V1 of the operational amplifier B1 and a level of an output voltage Vout of the power supply circuit. The smooth capacitor E1 has an electrostatic capacitance of a few μF (microfarad) order in order to smoothen the output voltage V1 of the operational amplifier B1.
In the power supply circuit A01 shown in FIG. 48, the output voltage Vout of the power supply circuit supplies power to functional blocks (loads) in a mobile device. For example, in the case of a liquid crystal display device comprising a power supply circuit, a control circuit, RAM and the like are connected to the power supply circuit output terminal 2. These functional blocks consume different levels of power in accordance with data displayed on a screen of the liquid crystal display device. Dividing the power consumption levels roughly into two categories, there are a state where large power (for example, a few-ten mw order) is consumed (large power consumption state) and a state where power is hardly consumed (momentary power consumption state). A current is supplied from the operational amplifier B1 via the resistance R01 in the large power consumption state, while a charge is supplied from the smooth capacitor E1 in the momentary power consumption state. Accordingly, stable power can be supplied to the control circuit, RAM and the like.
The following is needed in order to take power supply mode,                Though the charge can be momentarily supplied to the functional block demanding a high-speed processing, the smooth capacitor E1 is required.        It is necessary to use a resistance element having such a small resistance value (for example, at most a few-hundred Ω) that the output voltage Vout of the power supply circuit does not drop even in the large power supply as the resistance R01.        It is difficult to secure the phase stability because power is not supplied adversely in a less power consumption mode regardless of having such a capability that can respond to the large power in the operational amplifier B1.        In order to stabilize the phase, it is necessary to supply the current to an output transistor of the operational amplifier B1 or set the resistance R01 to a large value (for example, at least a few-hundred Ω).        
In order to realize the power supply circuit corresponding to the different modes, the value of the resistance R01 has different ideal values in accordance with the power consumption states, or it is necessary for the amplifier B1 to control so as to prevent the output transistor from oscillation even in the power-less mode. In either case, such disadvantages are generated that the output voltage Vout of the power supply circuit is shifted, phase allowance is reduced, the current of the output transistor is increased, and additionally chip area is increased.